Vascular diseases are often manifested by reduced blood flow due to atherosclerotic occlusion of vessels. For example, occlusion of the coronary arteries supplying blood to the heart muscle is a major cause of heart disease. Invasive procedures for relieving arterial blockage such as bypass surgery and stent placement with a catheter rely on estimates of occlusion characteristics and blood flow through the occluded artery. These estimates are based on measurements of occlusion size and/or blood flow. Unfortunately, current methods of occlusion size and blood flow measurement require invasive procedures such as coronary angiography, which requires cardiac catheterization. This procedure involves a long, thin, flexible catheter being placed into a blood vessel in the arm, groin (upper thigh), or neck; the catheter is then threaded to the heart. Through the catheter, a physician can perform a visual evaluation of the inner diameter of a vessel with cineangiography or fluoroscopy and/or use a small sensor on the tip of the wire (commonly a transducer) to measure parameters such as pressure, temperature, and flow to determine the severity of the lesion; and fractional flow reserve (FFR). These minimally invasive diagnostic tests on the heart carry the risk of stroke, heart attack, injury to the catheterized artery/heart, irregular heart rhythms, kidney damage, infection, and radiation exposure from X-rays. These procedures are time consuming, require expertise in the interpretation of the results and are expensive.
Stenosis geometry is also important in the therapeutic phase when balloon angioplasty, stenting or drug delivery procedures are subsequently performed. For example, precise stent placement is critical for reducing the risk of restenosis. Thus, decisions on whether or not to use any of the blockage relieving methods and which of the methods should be used are often based on partial information and do not take into account coronary collateralization. The ischemic stress often induces the increase in collateral circulation in coronary small vessel which at times will compensate for distal vessel blockage. Further, the evaluation of therapeutic success is also problematic, where both occlusion opening and stent position have to be evaluated. One class of methods, predominantly used today, require a lengthy procedure to find and determine severity, blockage to blood flow, of the lesion or lesions. Contemporary techniques evaluate the cardiac gradient phase-space changes and correlate the changes with cardiac computed tomography (CT), myocardial perfusion imaging, and cardiac angiography. The surface cardiac gradient contains detailed information on the electrophysiology of the chambers recorded. Because surface cardiac gradient represents the summation of the individual action potentials from each and every cardiac cell in syncytium, in theory, any information that might be determined from measurement of the orchestrated cellular action potential should be available on a “global” level in the surface. Moreover, although information relating to the influence of myocardial tissue architecture on conduction properties is inherent in the surface cardiac gradient, the challenge is in the discrimination of the pertinent information from these long quasi-periodic cardiac gradient signals while excluding noise contamination. Still further, there is a distinct lack of non-invasive tools available to enhance identification of high-risk patients and thus to trial preventive strategies in a non-invasive manner.